Semiconductor device including vanadium oxide sensor element with restricted current density

ABSTRACT

In a semiconductor device including a semiconductor substrate and at least one sensor element made of vanadium oxide formed over the semiconductor substrate, the sensor element is designed so that a density of a current flowing through the sensor element is between 0 and 100 μA/μm2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including asensor element made of vanadium oxide serving as a temperature sensor oran infrared sensor.

2. Description of the Related Art

Recently, temperature sensors have been introduced into a large scaleintegrated circuit (LSI) device to monitor the ambient temperaturethereof (see: JP-1-302849-A). As a result, when the ambient temperatureexceeds a normal temperature, a bias voltage applied to the LSI deviceis stopped to surely prevent the LSI device from being destroyedthermally due to the rise of the ambient temperature.

A first prior art temperature sensor is constructed by a temperaturesensor element formed by a diode and a resistor having differenttemperature coefficients (also see: JP1302849-A).

In the above-described first prior art temperature sensor, however,since the temperature coefficient of the diode is so small that thedifference in temperature coefficient between the diode and the resistoris small, a large signal-to-noise ratio (SNR) cannot be expected.

A second prior art temperature sensor is constructed by a temperaturesensor element formed by a parasitic bipolar transistor or a parasiticpn diode (see: JP-9-229778-A). Since the parasitic bipolar transistor orthe parasitic diode can be formed by a conventional MOS manufacturingprocess, the manufacturing cost can be decreased.

In the above-described second prior art temperature sensor, however,since the temperature coefficient of the parasitic bipolar transistor orthe parasitic diode is still small, for example, about 0.2 (%/K), alarge SNR cannot be expected.

A third prior art temperature sensor is constructed by a temperaturesensor element made of vanadium oxide (see: JP-11-330051-A). Since thetemperature coefficient of vanadium oxide is very large, a large SNR canbe expected.

Note that stable vanadium oxide is represented by a chemical formula VO₂or V₂O₅, generally, VO_(x) where X is about 2.

In the above-described third prior art temperature sensor, however,since a current is supplied to the vanadium oxide temperature sensorelement, when an excessive current is supplied thereto, the vanadiumoxide temperature sensor element would be broken down.

Also, JP-11-118567-A and JP-2003-121268-A disclose a bridge circuitincluding temperature sensor elements to enhance the sensitivity and atemperature detecting circuit connected to the two output terminals ofthe bridge circuit.

On the other hand, a prior art infrared sensor is constructed by aninfrared sensor element made of vanadium oxide (see: JP-2001-099705-A).Since the temperature coefficient of vanadium oxide is very large, alarge SNR can be expected.

Even in the above-described prior art infrared sensor, however, since acurrent is supplied to the vanadium oxide infrared sensor element, whenan excessive current is supplied thereto, the vanadium oxide infraredsensor element would be broken down. Note that the above-mentionedinfrared sensor is applied to an infrared camera formed by a pluralityof infrared sensors to which pulse-shaped currents are supplied. In thiscase, if the number of pixels is so small that the vanadium oxideinfrared sensor elements are large in size, such pulse-shaped currentssupplied thereto hardly break down the vanadium oxide infrared sensorelements. On the other hand, if the number of pixels is so large thatthe vanadium oxide infrared sensor elements are small in size, suchpulse-shaped currents supplied thereto would break down the vanadiumoxide infrared sensor elements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice including at least one sensor element made of vanadium oxidecapable of avoiding the breakdown thereof.

According to the present invention, in a semiconductor device includinga semiconductor substrate and at least one sensor element made ofvanadium oxide formed over the semiconductor substrate, the sensorelement is designed so that a density of a current flowing through thesensor element is between 0 and 100 μA/μm².

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from thedescription set forth below, with reference to the accompanyingdrawings, wherein:

FIG. 1A is a perspective view illustrating a sensor element made ofvanadium oxide according to the present invention;

FIG. 1B is a graph showing the length L-to-current I/W characteristicsof the sensor element of FIG. 1A;

FIG. 2 is a circuit diagram illustrating a first embodiment of thesemiconductor device including a temperature sensor element made ofvanadium oxide according to the present invention;

FIG. 3 is a cross-sectional view of the semiconductor device of FIG. 2;

FIG. 4 is a circuit diagram illustrating a second embodiment of thesemiconductor device including an infrared sensor element made ofvanadium oxide according to the present invention; and

FIG. 5 is a cross-sectional view of the resistor of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A sensor element made of vanadium oxide according to the presentinvention will be explained with reference to FIGS. 1A and 1B.

In FIG. 1A, a sensor element made of vanadium oxide has a length L, awidth W and a thickness T (=0.1 ∥m).

According to the inventors' experiments, when a voltage of 1V wasapplied between the terminals of the sensor element of FIG. 1A, acurrent I flowing therethrough was as shown in FIG. 1B. In FIG. 1B, thelarger the length L, the smaller the current I/the width W. In thiscase, when I(μA)/W(μm) was larger than 10, the sensor element brokedown. On the other hand, when I(μA)/W(μm) was smaller than 10, thesensor element of FIG. 1A did not break down. In other words, when acurrent density J(μA/μm²)=(I/W)/T(>0) was smaller than 100, the sensorelement of FIG. 1A did not break down. Thus, according to the presentinvention, the sensor element of FIG. 1A is designed so that a currentdensity J flowing therethrough is smaller than 100 μA/μm².

In FIG. 2, which illustrates a first embodiment of the semiconductordevice according to the present invention, this semiconductor deviceforms a temperature sensor which is constructed by a temperature sensingcircuit 1 and a differential amplifier 2.

The temperature sensing circuit 1 is constructed by sensor elements(temperature monitor resistors) R1 and R2 connected in series between apower supply terminal V_(cc) and a ground terminal GND and temperaturemonitor resistors R3 and R4 connected in series between the power supplyterminal V_(cc) and the ground terminal GND. Also, the temperaturemonitor resistors R2 and R3 are made of vanadium oxide whose temperaturecoefficient is −1.5(%/K), and the temperature monitor resistors R1 andR4 are made of amorphous silicon whose temperature coefficient is0.3(%/K). Further, assume that the resistance values of the temperaturemonitor resistors R1, R2, R3 and R4 are equal to 25 kΩ at a roomtemperature of 25° C. In this case, when the temperature is −50° C., theresistance values of the temperature monitor resistors R2 and R3 are 77kΩ, and the resistance values of the temperature monitor resistors R1and R4 are 20 kΩ, and when the temperature is 100° C., the resistancevalues of the temperature monitor resistors R2 and R3 are 8 kΩ and theresistance values of the temperature monitor resistors R1 and R4 are 30kΩ. Therefore, when the temperature is changed from −50° C. to 100° C.,the total resistance value of the temperature monitor resistors R1 andR2 and the total resistance value of the temperature monitor resistorsR3 and R4 are changed from 97 kΩ to 38 kΩ.

Also, assume that the power supply terminal V_(cc) is at 3.3V and theground terminal GND is at 0V, then, a current I₁ flowing through thetemperature monitor resistors R1 and R2 and a current I₂ flowing throughthe temperature monitor resistors R3 and R4 are changed from about 34 to87 μA.

If the width W and thickness T of the temperature monitor resistors R2and R3 is 20 μm and 0.2 μm, respectively, the cross-section of thetemperature monitor resistors R2 and R3 is 4 μm². Therefore, the currentdensity J flowing through the temperature monitor resistors R2 and R3 is8.5 to 21.8 μA/μm² which satisfies the above-mentioned condition thatthe current density J is between 0 and 100 μA/μm².

Also, the differential amplifier 2 amplifies the difference ΔV between asense voltage V₁ at a connection between the temperature monitorresistors R1 and R2 and a sense voltage V₂ at a connection between thetemperature resistors R3 and R4 to generate an output voltage V_(out)corresponding to the ambient temperature or the heat generated by aninternal logic circuit (not shown). In more detail, a p-channel load MOStransistor Q_(p1) and an N-channel MOS transistor Q_(n1) for receivingthe sense voltage V₁ of the temperature sensing circuit 1 are connectedin series between the power supply terminal V_(cc) and the groundterminal GND, and a p-channel load MOS transistor Q_(p2) and anN-channel MOS transistor Q_(n2) for receiving the sense voltage V₂ ofthe temperature sensing circuit 1 are connected in series between thepower supply terminal V_(cc) and the ground terminal GND. In this case,the transistors Q_(p1) and Q_(p2) form a current mirror circuit.

In FIG. 3, which is a cross-sectional view of the semiconductor deviceof FIG. 2, this semiconductor device is constructed by a p-typemonocrystalline silicon substrate 31, a multi-layer interconnectionlayer 32 formed on the silicon substrate 31, an insulating layer 33formed on the multi-layer interconnection layer 32 and a sheet layer 34formed on the insulating layer 33.

The multi-layer interconnection layer 32 is a stacked layer formed of aplurality of conductive layers and a plurality of insulating layers.

The sheet layer 34 is used only for covering the vanadium oxide, not forthe conventional conductive layers.

The semiconductor device of FIG. 3 is divided into a logic circuit areaAl for a logic circuit, the differential amplifier 2 of FIG. 2 and thelike and a temperature sensing area A2 for the temperature sensingcircuit 1 of FIG. 2.

In the logic circuit area A1, one typical complementary metal oxidesilicon (CMOS) transistor, i.e., one p-channel MOS transistor and onen-channel MOS transistor are formed. That is, an n-type well region 311with p⁺-type impurity diffusion regions 311S and 311D and a p⁻-type wellregion 312 with n⁺-type impurity diffusion regions 312S and 312D areformed in the silicon substrate 31. Also, gate electrodes G1 and G2, towhich voltages V_(G1) and V_(G2) are applied, are formed over then⁻-type well region 311 and the p⁻-type well region 312, respectively.Further, the p⁺-type impurity diffusion region 311S is connected througha via structure V11 to a wiring layer W11 which is also connectedthrough a via structure V12 to a wiring layer W12 to which the powersupply voltage V_(cc) is applied. The p⁺-type impurity diffusion region311D is connected through a via structure V13 to a wiring layer W13, andthe n⁺-type impurity diffusion region 312D is connected through a viastructure V14 to the wiring layer W13. The n⁺-type impurity diffusionregion 312S is connected through a via structure V15 to a wiring layerW15 which is also connected through a via structure V16 to a wiringlayer W16 to which the ground voltage GND is applied.

The temperature sensing area A2 is explained next.

The temperature monitor resistors R2 and R3 made of vanadium oxide(VO_(x)) are formed on the insulating layer 33 and are covered by thesheet layer 34. The temperature monitor resistors R2 and R3 have thesame rectangular shape of (10 to 100 μm)×(10 to 100 μm) with a thicknessof 0.1 to 0.2 μm. The temperature monitor resistors R2 and R3 have avolume resistivity of 0.01 to 10Ω·cm and a temperature coefficient of−1.5(%/K)

The temperature monitor resistors R1 and R4 made of n-type impuritiesdoped amorphous silicon are formed over the silicon substrate 31 withinthe multi-layer interconnection layer 32. The temperature monitorresistors R2 and R3 have the same rectangular shape of (1 to 100 μm)×(1to 100 μm) with a thickness of 0.1 to 0.3 μm. The temperature monitorresistors R2 and R3 have a temperature coefficient of 0.3(%/K). That is,the absolute value of the temperature coefficient of the temperaturemonitor resistors R1 and R4 is smaller than that of the temperaturemonitor resistors R2 and R3.

Note that the resistance values of the temperature monitor resistors R1,R2, R3 and R4 are approximately the same, for example, 25 kΩ) at a roomtemperature of 25° C.

A series of the temperature monitor resistors R1 and R2 is explainednext.

An end of the temperature monitor resistor R1 is connected through a viastructure V21 to a wiring layer W21 which is also connected through avia structure V22 to a wiring layer W22 to which the power supplyvoltage V_(cc) is applied.

The other end of the temperature monitor resistor R1 is connectedthrough a via structure V23 to a wiring layer W23 which is alsoconnected through a via structure V24 to a wiring layer W24 which isfurther connected through a via structure V25 to an end of thetemperature monitor resistor R2.

A p⁺-type impurity diffusion region 313 in the silicon substrate 31 isconnected through a via structure V26 to a wiring layer W26 which isalso connected through a via structure V27 to a wiring layer W27 whichis further connected through a via structure V28 to the other end of thetemperature monitor resistor R2.

A p⁺-type impurity diffusion region 314 is connected through a viastructure V29 to a wiring layer W29 which is also connected through avia structure V30 to a wiring structure W30 to which the ground voltageGND is applied.

Thus, the p⁺-type impurity diffusion region 313 is grounded by thep⁺-type impurity diffusion region 314 within the p-type siliconsubstrate 31.

The wiring layer W23 generates the sense voltage V₁ of FIG. 2.

Thus, the series of the temperature monitor resistors R1 and R2 areconnected between the power supply terminal V_(cc) and the groundterminal GND.

A series of the temperature monitor resistors R3 and R4 is explainednext.

An end of the temperature monitor resistor R3 is connected through a viastructure 31 to the wiring layer 22 to which the power supply voltageV_(cc) is applied.

An end of the temperature monitor resistor R4 is connected through a viastructure V32 to a wiring layer W32 which is also connected through avia structure V33 to a wiring layer W33 which is further connected tothe other end of the temperature monitor resistor R3.

Another end of the temperature monitor resistor R4 is connected througha via structure V35 to a wiring layer W35 which is also connectedthrough a via structure V36 to a p⁺-type impurity diffusion region 315which is grounded by the p⁺-type impurity diffusion region 314 in thesilicon substrate 31.

The wiring layer W32 generates the sense voltage V₂ of FIG. 2.

Thus, the series of the temperature monitor resistors R3 and R4 areconnected between the power supply terminal V_(cc) and the groundterminal GND.

In FIG. 3, the gate electrodes G1 and G2 are formed by the same layer.The temperature monitor resistors R3 and R4 are formed by the samelayer. The via structures V11, V13, V14, V15, V21, V23, V26, V32, V35and V36 are formed by the same material. The wiring layers W11, W13,W15, W21, W23, W26, W32 and W35 are formed by the same layer. The viastructures V12, V16, V22, V24, V27, V30 and V33 are formed by the samelayer. The wiring layers W12, W16, W22, W24, W27, W30 and W33 are formedby the same layer. The via structures V25, V28, V31 and V33 are formedby the same layer. The temperature monitor resistors R2 and R3 areformed by the same layer.

The operation of the semiconductor device of FIG. 2 is explained next.

As explained above, when V_(cc)=3.3V and GND=0V, the current 11 flowingthrough the temperature monitor resistors R1 and R2 and the current I₂flowing through the temperature monitor resistors R3 and R4 are from 34to 87 μA, and the current density J of the temperature monitor resistorsR2 and R3 is from 8.5 to 21.8 μA/μm². As a result, the sense voltage V₁at the connection between the temperature monitor resistors R1 and R2 isdetermined by V_(cc)×R2/(R1+R2), and the sense voltage V₂ at theconnection between the temperature monitor resistors R3 and R4 isdetermined by V_(cc)×R4/(R3+R4).

When the ambient temperature is increased or the logic circuit area A1generates heat therein, the temperature monitor resistors R1, R2, R3 andR4 are also heated. Therefore, the decrease in the resistance values ofthe temperature monitor resistors R2 and R3 is relatively large, whilethe increase in the resistance values of the temperature monitorresistors R1 and R4 is relatively small. As a result, the sense voltageV₁ is decreased and the sense voltage V₂ is increased, so that thedifference between the sense voltage V₁ and V₂ is enhanced. Thus, theenhanced difference between the sense voltages V₁ and V₂ is amplified bythe differential amplifier 2 to generate its output voltage V_(out). Forexample, if the output voltage V_(out) is larger than a predeterminedvalue, the bias voltage applied to the logic circuit area A1 of FIG. 3is stopped.

In FIGS. 2 and 3, since the current density J of the temperature monitorresistors R2 and R3 is smaller than 100 μA/μm², the temperature monitorresistors R2 and R3 can be prevented from being destroyed.

Also, since the temperature monitor resistors R2 and R3 are made ofvanadium oxide having a large temperature coefficient, i.e., about1.5%/K, a higher SNR can be obtained.

Further, the temperature sensing circuit 1, i.e., the bridge circuitformed by the temperature monitor resistors R1, R2, R3 and R4 canincrease the accuracy of measurement of temperature, and thedifferential amplifier 2 amplifies the difference in output between thebridge circuit can further increase the accuracy of measurement oftemperature.

For example, at a room temperature of 25° C., if each of the temperaturemonitor resistors R1, R2, R3 and R4 is 1 kΩ, the sense voltages V₁ andV₂ under V_(cc)=3.3V and GND=0V are

$\begin{matrix}{V_{1} = {3.3 \times {1/\left( {1 + 1} \right)}}} \\{= {1.65\mspace{14mu} V}}\end{matrix}$ $\begin{matrix}{V_{2} = {3.3 \times {1/\left( {1 + 1} \right)}}} \\{= {1.65\mspace{14mu} V}}\end{matrix}$ $\begin{matrix}{{\therefore{\Delta \; V}} = {V_{2} - V_{1}}} \\{= {0\mspace{14mu} V}}\end{matrix}$

Then, the temperature is increased by 10° C. to 35° C.,

$\begin{matrix}{{R\; 2} = {R\; 3}} \\{= {{\left( {1 - {0.015 \times 10}} \right) \cdot 1}\; k\; \Omega}} \\{= {0.85k\; \Omega}}\end{matrix}$ $\begin{matrix}{{R\; 1} = {R\; 4}} \\{{= {{\left( {1 + {0.003 \times 10}} \right) \cdot 1}k\; \Omega}}\;} \\{{= {1.03k\; \Omega}}\;}\end{matrix}$ $\begin{matrix}{{\therefore V_{1}} = {3.3 \times {0.85/\left( {0.85 + 1.03} \right)}}} \\{= {1.49\mspace{14mu} V}}\end{matrix}$ $\begin{matrix}{V_{2} = {3.3 \times {1.03/\left( {0.85 + 1.03} \right)}}} \\{= {1.81\mspace{14mu} V}}\end{matrix}$ $\begin{matrix}{{\Delta \; V} = {V_{1} - V_{2}}} \\{= {{- 0.32}\mspace{14mu} V}}\end{matrix}$

Note that the differences ΔV is twice the corresponding differencewithout the bridge circuit. This difference is further increased by thedifferential amplifier 2.

In FIG. 3, the temperature monitor resistors (vanadium oxide) R2 and R3are formed on the insulating layer 33 under the sheet layer 34 and thelogic circuit of the logic circuit area A1 within the silicon substrate31 and the multi-layer interconnection layer 32. Therefore, the logiccircuit and the manufacturing equipment therefor is hardly contaminatedby vanadium oxide. Note that the temperature monitor resistors(amorphous silicon) R1 and R4 are manufactured by a conventionalmanufacturing process.

Further, the temperature monitor resistors R1 and R4 can be made ofpolycrystalline silicon.

Additionally, in FIG. 3, a part of the temperature monitor resistors R2and R3 can be formed on the logic circuit area A1, which would enhancethe integration.

The temperature sensing circuit 1 and the differential amplifier 2 ofFIG. 2 can be at a single place or a plurality of places of one chip. Inthis latter case, a mean value of temperature measurements can be used.

In FIG. 4, which illustrates a second embodiment of the semiconductordevice according to the present invention, this semiconductor deviceforms an infrared detecting sensor mounted on an infrared receiving unitof an infrared camera.

In FIG. 4, one pixel P_(ij) (i=1 , 2, . . . , m, j=1, 2, . . . , n)formed by a MOS transistor Q and an infrared monitor resistor R isprovided at each intersection between gate lines X₁, X₂, . . . . X_(m)and data lines Y₁, Y₂, . . . , Y_(n). The gate lines X₁, X₂, . . . ,X_(m) are sequentially selected by a vertical shift register 41 whichreceives a vertical synchronization signal VSYNC and shifts it inaccordance with a horizontal synchronization signal HSYNC. On the otherhand, the data lines Y₁, Y₂, . . . , Y_(n) are sequentially selected bya horizontal shift register 42 which receives the horizontalsynchronization signal HSYNC and shifts it in accordance with a clocksignal CLK. Thus, in a selected pixel P_(ij), a signal voltage V_(out)is dependent upon its bias current I_(B) which is dependent upon theresistance of the infrared monitor resistor R of a selected pixel.

In FIG. 5, which is a cross-sectional view of the infrared monitorresistor R of FIG. 4, reference numeral 51 designates a p-typemonocrystalline silicon substrate on which a reflection plate 52 made ofCr/Pt is formed. Also, an insulating layer 53 made of SiN, a pair ofconductive beams 54 and 55 made of NiCr and an insulating layer 56 madeof SiN are formed, and an infrared sensor element 57 made of vanadiumoxide connected between the conductive beams 54 and 55 are provided. Inthis case, the infrared sensor element 57 is also supported by theconductive beams 54 and 55, so that an air gap AG is formed. Thus, theinfrared sensor element 57 is isolated thermally from the siliconsubstrate 51.

The infrared sensor element 57 has a length L of 20 μm, a width W of 2μm and a thickness T of 0.2 μm with a cross section of 0.4 μm².

The cross section of the infrared sensor element 57 is determined sothat the current density J is between 0 and 100 μA/μm². For example,when no infrared is incident to the infrared sensor element 57, theresistance value of the infrared sensor element 57 is 120 kΩ, and wheninfrared is incident to the infrared sensor element 57, the resistancevalue of the infrared sensor element 57 is 95 kΩ. Since the power supplyvoltage V_(cc) is 3.3V and the ground voltage GND is 0V, the currentflowing through the infrared sensor element 57 is 27 to 35 μA at most,which corresponds to a current density of 67 to 88 μA/μm². Thus, thecurrent density can be smaller than 100 μA/μm², so that the infraredsensor element 57 made of vanadium oxide would hardly breakdown.

When infrared is incident to the device as indicated by arrows in FIG.5, a part of the infrared is absorbed directly by the infrared sensorelement 57, so that the temperature thereof would be increased.Simultaneously, if the infrared passes through the infrared sensorelement 57 and the air gap AG to reach the reflection plate 52, thispart of the infrared is reflected by the reflection plate 52 and reachesthe infrared sensor element 57. Thus, the temperature of the infraredsensor element 57 would be further increased.

The above-mentioned infrared sensor element 57 is supplied with apulse-shaped bias current IB by the vertical shift register 41 and thehorizontal shift register 42 of FIG. 4. Measurements of the bias currentIB are carried out by a control circuit (not shown) to form an infraredimage.

As explained hereinabove, according to the present invention, thecurrent density of an infrared sensor element made of vanadium oxide isbetween 0 and 100 μA/μm², the breakdown of a sensor element such as atemperature monitor resistor and an infrared sensor element can beavoided.

1. A semiconductor device comprising: a semiconductor substrate; and atleast one sensor element made of vanadium oxide formed over saidsemiconductor substrate, said sensor element is designed so that adensity of a current flowing through said sensor element is between 0 to100 μA/μm².
 2. The semiconductor device as set forth in claim 1, whereinsaid sensor element is configured so that a resistance of said sensorelement is changed in accordance with an electromagnetic wave incidentto said device.
 3. The semiconductor device as set forth in claim 2,wherein said electromagnetic wave is an infrared ray.
 4. Thesemiconductor device as set forth in claim 2, further comprising areflection plate made of Cr/Pt provided on said semiconductor substrate.5. The semiconductor device as set forth in claim 1, comprising aplurality of said sensor elements which are arranged in a matrix.
 6. Thesemiconductor device as set forth in claim 5, further comprising: aplurality of MOS transistors each connected to one of said sensorelements; a plurality of gate lines; a plurality of data lines; avertical shift register, connected to said gate lines, for sequentiallyselecting said gate lines; and a horizontal shift register, connected tosaid data lines, for sequentially selecting said data lines, one of saidMOS transistors and one of said sensor elements being connected at eachintersection between said gate lines and said data lines.